Because of its light weight and good workability, a synthetic high molecular material finds wide application. In general, the synthetic high molecular material has a specific volume resistivity as high as 10.sup.15 to 10.sup.17 .OMEGA..multidot.cm and thus can easily be statically electrified. Thus, the synthetic high molecular material causes serious troubles. For example, various electrostatic troubles occur. Further, the molded product of the synthetic high molecular material attracts dust on the surface thereof, marring the appearance thereof. Therefore, in an attempt to provide these synthetic high molecular materials with antistatic properties, various methods have heretofore been tried. The method for providing a resin with antistatic properties can be roughly divided into the following three groups:
(1) A method which comprises applying a surface active agent to the surface of a resin so that the hydrophilic group in the surface active agent adsorbs water in the atmosphere to form a low resistivity electrically-conductive layer on the surface of the molded resin product; PA0 (2) A method which comprises mixing a low molecular surface active agent in a resin so that the surface active agent bleeds out to the surface of the molded resin product during or after molding, allowing the hydrophilic group in the surface active agent to adsorb water in the atmosphere and hence forming a low resistivity electrically-conductive layer on the surface of the molded resin product; and PA0 (3) A method which comprises alloying the desired resin with an ionically-conductive polymer so that a thin electrically-conductive polymer phase layer is formed in the vicinity of the surface layer during molding, allowing the polymer phase layer to adsorb water in the atmosphere and hence forming an electrically-conductive layer.
Most common among the foregoing three methods is method (2). This method can work with the addition of an antistatic agent in an amount as small as 0.05 to 2% by weight. This method also can apply to molded products having various shapes. Further, this method essentially involves the use of a low molecular compound which can migrate through a resin. Therefore, the active agent which has bled out to the surface of the molded product can easily be removed when the surface of the molded product is cleaned or wiped out. As a result, the antistatic properties of the molded resin product are lost and take a long time to restore. In this sense, the foregoing antistatic agent is referred to as the same "nonpermanent antistatic agent" as that in method (1). Molded products having a great specific surface area such as sheet and film require a drastically increased amount of an antistatic agent. Therefore, this method can be mainly applied to resins having a glass transition point of not higher than room temperature.
In other words, this method can mainly apply to polypropylene and polyethylene and partly to soft polyvinyl chloride, etc. but can exert only an extremely low effect with resins having a high glass transition temperature such as polystyrene, ABS resin, poly(methyl methacrylate), soft polyvinyl chloride and polycarbonate.
Method (3) has been developed as a process for the antistatic treatment of these high glass transition resins. This method can exert a sufficient antistatic effect only when an expensive ionically-conductive polymer is used in an amount of from 20 to 30 vol-% on the basis of percolation theory.
As mentioned above, any of methods (1) to (3) involves ionic conduction by adsorbed or intercalated water as an essential electrical conduction mechanism. Thus, these methods have a common disadvantage in that the antistatic effect thus developed is drastically impaired under dried conditions or in the winter season.
On the other hand, in an attempt to modify a resin, the dispersion of an organically-modified layer silicate in which an organic compound is intercalated has been tried. A layer silicate is a typical layer inorganic compound constituting clay. For example, a 2:1 type lamellar silicate mineral comprises two sheets of silica tetrahedral silicate having an octahedral sheet containing magnesium or aluminum sandwiched therebetween. These three sheets constitutes one indivisible silicate layer (thickness: 1.0 nm). Several to scores of sheets of these silicate layers are laminated in parallel to form a primary aggregate. In general, these primary aggregates of layer silicate are further aggregated randomly to form a secondary aggregate having a particle diameter of hundreds of nanometers to several micrometers. Smectite, vermiculite, talc and mica are typical layer silicate compounds having such a structure. Among these layer silicate compounds, smectite and synthetic mica have a proper interlayer charge density and hence exhibit good water swelling characteristics. It is thus said that when immersed in water, these layer silicate compounds can dissociate its secondary aggregate structure and can thus be uniformly dispersed. Some kinds of layer silicate compounds can even dissociate the rigid primary aggregate structure and can thus be dispersed in the form of single layer ("Handbook of Clay", 2nd ed., Japan Society of Clay). These lamellar silicate compounds have an ion exchange capacity. When brought into contact with various cationic compounds, a composite material comprising smectite or synthetic mica having these cationic compounds intercalated therein rather than metallic ions which have heretofore been used. In this case, if an organic ammonium salt as an organic cation is used, a lipophilic (hydrophobic) organically-modified layer silicate can be obtained (The term "organically-modified" as used in this case is meant to indicate the state of modification involving the exchange of ions between layers rather than mere surface modification or surface treatment of aggregate with a coupling agent or active agent). These organically-modified layer silicates occur in the form of secondary aggregate similar to the foregoing unmodified layer silicate. In other words, these organically-modified layer silicates have an increased spacing (including the thickness of one silicate layer) of from several nanometer to 7.0 nm depending on the structure of organic ammonium intercalated between layers. However, these organically-modified layer silicates essentially comprise a regular layer primary aggregate structure which is randomly aggregated to form a secondary aggregate. A smectite clay mineral which has been organically modified, particularly with dodecylammonium, octadecylammonium, trimethyloctadecylammonium, dimethyldioctadecylammonium, benzyldimethyloctadecylammonium, etc., has been marketed in the name of organic bentonite and long been used as coating thickener. These known organically-modified layer silicates have been available in the form of compound modified with a single or double long-chain (other substituents on nitrogen atom include hydrogen atom, methyl group and benzyl group) alkylammonium having 18 or less carbon atoms for the convenience of synthesis of ammonium salt. These organically-modified layer silicates are soluble in some aromatic solvents such as toluene and benzene but have a low affinity for polar solvents such as alcohol and acetone and thus are insoluble in aliphatic hydrocarbon solvents such as hexane and pentane.
Various attempts have been made to disperse these organically-modified layer silicates in a resin. However, these organically-modified layer silicates can be dispersed in a resin more hardly than in organic solvents due to its dependence of solubility parameter on molecular weight. In general, even a secondary aggregate cannot be dissociated. Such a poor resin dispersion system cannot exert an antistatic effect as described in Comparative Example 3 in JP-A-61-213231 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). In particular, a thermoplastic resin having a low polarity such as polyolefin thermoplastic resin and polystyrene thermoplastic resin can even more hardly be uniformly dispersed in an organically-modified clay.
A single long-chain organically-modified layer silicate such as dodecyltrimethyl ammonium-modified clay and trimethyl octadecyl ammonium-modified clay has a low affinity for organic materials and thus is unevenly present almost as it is in the form of secondary aggregate having a size of several micrometer to scores of micrometer in a resin such as polystyrene and polypropylene. Such a huge secondary aggregate of lamellar silicate in a resin can easily be visually recognized. Even a microscopically transparent resin looks cloudy. When analyzed by electron microscope or like means, such a resin dispersion can be confirmed to have no silicate layers dispersed in the resin matrix (FIG. I-1). The double long-chain dimethyldioctadecyl ammonium-modified clay, which has been most widely spread, provides some improvement in the compatibility with resins. However, the double long-chain dimethyldioctadecyl ammonium-modified clay leaves something to be desired. For example, a product obtained by melt-kneading the modified clay with a polypropylene resin by means of a roll kneader for 10 minutes has huge secondary aggregates having a thickness of not less than 1 .mu.m left therein ("Transactions of 4th Forum of Polymer Materials", page 294, 1995). It is known that when the modified clay is mixed in a solvent in a higher mixing efficiency, only an organic clay-resin composition having a drastically reduced transparency in other words, the content of aggregates having a short axis length of not less than about wavelength of visible light (=1 .mu.m) is very high! can be obtained ("Transactions of 38th Forum of Clay Science", page 52, 1994).
Such a poor dispersion system cannot exert an antistatic effect as described in Comparative Example 3 in JP-A-61-213231.
The above cited JP-A-61-213231 proposes a technique which comprises adding an organically-modified clay to a resin composition containing an organic antistatic agent to inhibit the bleeding of the organic antistatic agent and hence provide a prolonged stability of antistatic properties. As a system which shows no resistivity drop there is disclosed in Comparative Example 8 in the above cited patent a composition system made of trimethyloctadecyl ammonium-modified clay and a polyvinyl chloride (containing a large amount of a plasticizer) having a spcific volume resistivity of 7.times.10.sup.11 .OMEGA..multidot.cm. In the comparative example, an organically-modified clay is used in an amount as large as 40 parts by weight based on 100 parts by weight of the resin used. It is obvious that such an ordinary composition system exceeding the threshold value of percolation shows some resistivity drop. Thus, such a composition system has neither technical significance nor industrial usefulness. Further, such a composition system essentially differs from the target aimed by the inventors.
JP-A-58-67338 discloses a gel-forming agent comprising an organic cation-organic anion complex intercalated in a smectite type clay. It is proposed that the gel-forming agent may comprise a branched chain incorporated therein as an organic cation which is one component of the complex. However, the gel-forming agent is quite different in structure from the organically-modified layer silicate comprising only a quaternary ammonium cation having a specific branched chain intercalated therein as defined herein. Further, this patent discloses no examples of gel-forming agent having a branched chain. Moreover, no reference is made to the fact that the gel-forming agent described in this patent can be dispersed in a nonpolar solvent such as liquid paraffin. There is no suggestions to the fact that thermoplastic resins can be provided with permanent antistatic properties.
The inventors have made extensive studies of the foregoing problems. As a result, a process for the permanent antistatic treatment of a resin based on quite a new mechanism has been found. In other words, it has been found that a resin comprising a properly organically-modified lamellar silicate compound having a specific volume resistivity as extremely low as from 10.sup.8 to 10.sup.12 .OMEGA..multidot.cm due to electronic conduction uniformly dispersed therein exhibits permanent antistatic properties. In some detail, an organically-modified layer silicate compound having a specific volume resistivity of not more than 1.times.10.sup.13 .OMEGA..multidot.cm is dispersed in a synthetic resin in such an arrangement that a primary aggregate and/or a secondary aggregate having a short axis length of not more than 500 nm is formed and the average minimum interparticle distance is not more than 500 nm. In accordance with the present process, various resins can be provided with permanent antistatic properties which cannot be impaired even when the surface of the molded product of the resins is wiped. This process can apply to non-polar polymers such as polyolefin resin. The antistatic properties appear immediately after molding. The antistatic properties don't need to be mediated by adsorbed water and thus can work even under low temperature and low humidity conditions. The present process also makes it possible to uniformly reduce the specific volume resistivity of the entire resin and thus is also effective for both thick and film-like products and film-like molded products having a great specific surface area. Thus, any special molding conditions under which the antistatic properties can work are not required. The present process can apply to a system having a filler or a third component incorporated therein without any problem. The present process can further apply to a system in which a highly-concentrated master batch is diluted before use without any problem. Further, the present process neither deteriorates the physical properties of the resin nor impairs the color tone of the resin. The present process can maintain the transparency of the resin, if it is transparent. It should be noted that the permanent antistatic treatment technique disclosed herein makes it possible to reduce the lowest necessary added amount of the organically-modified layer silicate compound as an electrically conductive carrier to a value as extremely small as 2 to 3 vol-%. Further, the permanent antistatic resin composition prepared according to the process of the present invention exerts secondary effects. In other words, the permanent antistatic resin composition of the present invention exhibits excellent weather resistance, heat resistance, dimensional stability, corrosion resistance, abrasion resistance, fire retardance and gas barrier properties.
The inventors further found that the novel organically-modified layer silicate which can be preferably used in the foregoing process can also exert an excellent effect as a thickening agent for non-polar solvent. Thus, the present invention has been worked out.